With 50% duty cycle: in fact the time at high state depends of the bottom left resistor (R2) and the capacitor, and the time at low state depends of the top left resistor (R1) and the capacitor
Now the maths:
Th = - ln(1 + (Vcc/3) / (Vd - (2Vcc/3))) * R2 * C
Tl = - ln(1 + (Vcc/3) / ((2Vcc/3) - Vd)) * R1 * C
Duty cycle = Th / (Th + Tl)
Th: time at high state
Tl: time at low state
Vd: diode voltage
